We established that 16 mM aspirin could be used to stop further prostaglandin (PG) production by rat tissues and that measuring PG content by radioimmunoassay was parallel with recoveries of 95% plus or minus 10%. Incubation time courses of PG synthesis were constructed for the clotting of blood, and heart, lung, liver and kidney meddula homogenates and adipocytes. Procedures for measuring factor 3 activity and lipolysis were also validated. Rats were fed diets containing either 20% or 40% fat calories and P/S ratios of 0.2,0.4 or 3.0 for 1 or 4 months. The hypothesis was that increased dietary polyunsaturated fatty acids would be reflected in endogenous lipid composition and thus alter PG profiles. Only a very short incubation time in liver homogenates and serum (i.e., platelets) incubations between 10 and 40 minutes showed a positive correlation (P less than 0.05) between amount of dietary polyunsaturated fatty acids and total PGs produced.